Tetraethyl orthosilicate is a major chemical compound with a formula Si(OC2H5)4. The compound is frequently abbreviated as TEOS and consists of four ethyl groups attached to an SiO4 ion that is referred to as an ortho silicate. TEOS can also be considered to be the ethyl ester of orthosilicic acid, Si(OH)4 and is a prototypical alkoxide. TEOS is a tetrahedral molecule with many analogues, most of which are prepared by alcoholysis of silicon tetrachloride or the direct reaction of silicon metal and ethanol.
Typically TEOS is hydrolyzed and condensed with mineral acid catalysts. TEOS is mixed with sufficient alcohol to allow its reactant water to be partially miscible in the presence of an acid catalyst. This allows the initial reaction to take place where one of the ethoxy groups is replaced by a water molecule liberating ethanol as a by-product. This is illustrated by the following diagram:Si(OC2H5)4+H2O→(C2H5O)3SiOH+C2H5OH
The condensation of silanol or triethoxysilanol (RO)3SiOH, wherein R is C2H5, derived from the hydrolysis reaction is a competing reaction in the presence of acid catalyst. This is illustrated by the following diagram:(C2H5O)3SiOH+HOSi(OC2H5)3→(C2H2O)3SiOSi(OC2H5)3+H2O
Thus, the polymerization of TEOS with water in the presence of a catalyst results from sequential hydrolysis of ethoxy groups and condensation of silanol in this over simplified view. In fact, there are other reactions, such as condensation of silanol with ethoxy groups that provide equivalent polymerization. All these processes result in polymers of increasing complexity, variety in physical properties, and usefulness in their intended applications.
Typically these polymers are thought of as combinations of linear, cyclic and polycyclic polymers. Physical properties such as dynamic viscosity, viscosity stability, and available silica (SiO2 wt %), are dependent on the amount of water reacted with the TEOS. This is expressed as a percentage of the theoretical amount of water to replace all the ethoxy groups bound to silicon. The stoichiometric amount of water is 2 moles of water to 1 mole of TEOS. This is referred to as 100% hydrolysis. When reacted with this amount of water, the resulting polymer has the physical properties of amorphous silica. This can be observed by reacting TEOS with 2 moles of water and then removing the by-product ethanol by distillation. What remains is a solid material with a high percentage of SiO2 remaining in the composition, typically 98% or higher. In practice it has been found that in order to obtain usable physical properties, such as a flowable liquid, only 40% of the theoretical water is added and then the ethanol byproduct is removed. This results in a low viscosity liquid ethylsilicate polymer, or ethylpolysilicate, that contains 40 wt % available SiO2. The end product polymer is considered to be a 40% hydrolyzed ethylsilicate polymer This material has become an item of commerce, known as SILBOND™ 40 or Dynasil® 40.
TEOS also has many applications because of its easy conversion into SiO2. For example, it may be used for chemical mechanical polishing or the synthesis of low molecular weight trimethylsiloxypolysilicates (MQ Resins) that are useful as additives for release coatings in pressure sensitive adhesive applications and liquid silicone rubber. The role of MQ Resins in such formulations is to modify the properties of the cured silicone rubber. The silicone rubber is hardened by the MQ resin to a higher modulus. For pressure sensitive release applications MQ resin increases the release forces of the adhesive. In both applications, the presence of the low molecular weight MQ Resins is detrimental.
A further application for TEOS, where polymer distribution with reduced amounts of low molecular weight material are desired, relates to the formation of polysilicate binders for zinc rich coatings, investment casting, refractory, sand core, and ceramic articles in general. A developing limitation for all these applications is the amount of low boiling alcohol by-product, referred to as low boilers, liberated during the further hydrolysis of the ethylsilicate polymer. This raises the amount of volatile organic components (VOCs) present in the formulation. More environmentally friendly compositions are desired.